Electroluminescent devices such as light emitting diodes (LEDs) have in recent years included the use of organic materials as one or more of the device components. Organic materials are desirable for their light weight and low cost. Unfortunately, many organic materials suffer from low stability and low durability as compared with metallic materials.
Electroluminescent devices (ELDs) are typically constructed with two electrodes. An electroluminescent material is in electrical contact with both electrodes, and forms a conduction path between the electrodes. One electrode functions as a electron-injection layer, while the other electrode functions as a hole-injection layer.
High efficiency PLED/OLED devices rely on a cathode which has sufficiently low work function so that electron can be injected into the polymer easily. Traditional approach uses low work function metals, such as calcium and barium metal, to facilitate the efficient electron injection into the light-emitting polymers. Since these metals are chemically very reactive, they can be easily destroyed by trace of moisture from air. In order for the device to have a reasonably long lifetime, these devices not only need to be fabricated and encapsulated in an inert gas environment, a desiccant is usually packaged internally to extend the device life (see drawing below). Even though, the current lifetime of a full color OLED display is still <10,000 hours, which is insufficient for many applications. The decay of cathode is proven to be one of the several key factors that have significant influence on the device life. Therefore, development of environmentally stable and low work function cathode for OLED remaining an important topic in OLED industry.
In addition, the use of internal desiccant not only increases the complexity and cost of manufacturing, it also increases the weight and the dimension of the display product. In certain applications requires harsh environment and/or strong mechanical vibrations, the desiccant approach may not be viable. This is because large change of temperature and/or strong vibration may cause stripping of the desiccant, which may result in device failure. Thus, development of more stable cathode remains one of the key issues towards the commercialization of OLED displays.
The use of more stable metals such as Aluminum, copper, gold, etc., can improve the resistance of the cathode to moisture and oxygen, thus potentially eliminating the need of desiccant and reducing the requirement for hermetic encapsulation. However, the work function of these metals is usually too high to match the LUMO orbital of the light-emitting polymer (LEP) and results in a large energy gap for electron injection into the LUMO of the LEP.
There remains a need in the art to overcome these drawbacks, as well as generally to develop new methods and materials for ELDs. Ideal ELDs would be robust and efficient, with an extended operational lifespan. An ideal method for preparing ELDs would utilize materials that are readily available or easily prepared, would minimize the number of process steps, and would provide highly reproducible results.